Why do light emitting diodes are preferred over conventional incandescent low power lamps?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → The bandwidth of emitted light is 10 nm to 50 nm
A light emitting diode (LED) is a heavily doped, forward-biased p-n junction that emits light when electrons and holes recombine.
Option (1) High operational voltage and less power — Incorrect. An LED works at a low operational voltage, not a high one; only the "less power" part is right.
Option (2) Short life and ruggedness — Incorrect. An LED has a long life; short life would be a disadvantage, not a reason to prefer it.
Option (3) The bandwidth of emitted light is 10 nm to 50 nm — Correct. The light of an LED has a bandwidth of about 100 Å to 500 Å, that is 10 nm to 50 nm, so it is nearly monochromatic, though not perfectly, unlike the broad spectrum of a hot filament.
Option (4) Warm up time required — Incorrect. An LED has fast action and needs no warm-up time; it is the filament lamp that takes time to heat up.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic was removed from the latest NCERT book (2026-27), and it also appears to be out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Optoelectronic Junction Devices – “… LEDs have the following advantages over conventional incandescent low power lamps:
(i) Low operational voltage and less power.
(ii) Fast action and no warm-up time required.
(iii) The bandwidth of emitted light is 100 Å to 500 Å or in other words it is nearly (but not exactly) monochromatic.
(iv) Long life and ruggedness.
(v) Fast on-off switching capability.”